Electron spin resonance study of methyl‐substituted N‐hydropyridinyl and N, N′‐dihydrodiazinyl radicals in solution

ESR spectra of a number of methyl‐substituted N‐hydropyridinyl, N, N′‐dihydropyrazinyl, and N, N′‐dihydropyrimidinyl radicals have been measured in solution at room temperature. The radicals were produced by in‐situ photolytic reduction of the parent azaaromatic compounds. The hyperfine coupling constants have been analyzed in terms of the Karplus‐Fraenkel relation and the McConnell‐type relations for the CH, CH3, and NH proton splittings and using the condition for conservation of π‐spin population. Consistent values of the spin polarization parameters QN, QNCN, QHCH, and QHCCH3, but not of QHNH, were obtained virtually independently of calculated spin populations. Experimental and calculated π‐spin populations have been compared for the N‐protonated azaaromatic radicals from this and other work. There is no correlation between the NH proton coupling constant and the experimental π‐spin population on the nitrogen atom in substituted N‐hydropyridinyl radicals. The non‐transferability of the McConnell‐type parameter QHNH is attributed to a contribution of the out‐of‐plane vibration of the NH proton.